The present invention relates to prostheses for replacement of a portion of the shoulder joint. More particularly, the present invention relates to a modular prosthesis for replacement of the upper portion of the humerus.
Conventional prostheses for the replacement of the upper portion of the humerus are typically unitary structures, generally including a stem designed to extend downwardly into a cavity within the humerus. The stem of the conventional prostheses is secured within the bone of the humerus by bone cement or through the use of coatings that promote bone ingrowth to secure the stem. The stem is attached to a body portion designed to replace portions of the humerus in the vicinity of the anatomical neck of the humerus. The conventional shoulder prosthesis also includes a generally spherical head portion configured to replace the head of the humerus. A collar is generally provided between the head portion and the stem to stabilize the prosthesis on the resected humerus. Generally, the collar rests on the resected surface of the humerus to distribute imposed loading on the prosthesis over a greater area of bone.
One difficulty with conventional unitary shoulder prostheses is the necessity of maintaining large inventories of differently sized prostheses to accommodate the different bone sizes of prospective patients. With such unitary shoulder prostheses, the patient is typically evaluated by x-ray to determine the approximate prosthesis size needed for bone replacement. A number of differently sized prostheses are selected as possible candidates for bone replacement on the basis of this evaluation, and the final selection of the appropriate prosthesis is selected during the replacement operation.
Fully or partially modular prostheses that diminish the problem of maintaining a large inventory are known for use as femur replacements. For example, U.S. Pat. No. 4,051,559 discloses a prosthesis that includes a separate threaded stem that is adapted to be screwed into a prepared cavity within the femur. The prosthesis separately comprises a head portion that includes a neck and collar that is adapted to be mated with the stem. This prosthesis is not designed to be assembled prior to insertion within the patient. The stem must first be screwed into the cavity within the femur. The head portion is then attached to the installed stem by a bolt. The collar is designed to rest upon the resected surface of the femur to support the load placed on the prosthesis. Because of the design of the stem, the head portion must include the collar in order to support the weight that will be placed on the prosthesis. The modularity of this prosthesis is limited because the neck and collar is a unitary structure.
Another prosthesis having a stem and a separate head portion is shown in U.S. Pat. No. 3,806,957, which discloses a prosthesis that includes a separate stem having a broadened proximal end. The broadened proximal end of the stem is configured to receive a head and threaded neck portion to form a complete prosthesis. The neck may be elongated or shortened depending on the specific anatomy of different patients. This prosthesis, like the prosthesis disclosed in U.S. Pat. No. 4,051,559, is limited in flexibility because the head and neck portion is a unitary structure, and because the head and neck portion attaches directly into the stem.
A prosthesis in which the stem comprises more than one component is shown in U.S. Pat. No. 3,987,499. U.S. Pat. No. 3,987,499 discloses a prosthesis having a stem or shank component that includes two parts, an anchoring part and a transition part. A ball is connected to the transition part. Also, a collar may be included between the ball and a portion of the femur. The anchoring part is provided with external threads that are adapted to tap themselves into the femur. The transition part is coupled to the anchoring part by a guide pin and securing screw. The ball is adapted to be screwed onto the free end of the transition part. The prosthesis is designed to be placed in position within the body component by component, and assembled sequentially.
In addition to modular femoral prostheses, partially modular humeral prostheses have been described. For example, U.S. Pat. No. 4,865,605 to Dines et al. discloses a shoulder prosthesis having a humeral component and a glenoid component. The humeral component includes a stem and an integrally attached collar. A spherical head having a stem fittable into a tapered bore in the stem of the humeral component to provide a friction tight attachment between the head and the stem of the humeral component is also described. Both the head and the stem of the humeral component are available in different sizes.
According to the present invention, a kit for the assembly of a shoulder prosthesis for the replacement of a portion of a humerus is provided. The kit includes at least two stems of differing size, with each stem sized for insertion into the humerus and at least two bodies, with each body sized to replace a portion of the humerus. In addition, the kit includes at least two head members, with each head member sized to replace the head of the humerus. Attachment means for fixedly attaching one of the at least two head members to one of the at least two bodies is also provided. Optionally, the kit can contain at least two collars, with each collar configured to be positioned between one of the at least two head members and one of the at least two bodies to contact humeral bone and stabilize the position of the inserted shoulder prosthesis.
Preferably, the kit of the present invention consists of a plurality of stems, bodies, collars, and head members all having various sizes and shapes. These separate components are adapted to be assembled together to form a custom prosthesis of a desired size and shape. One advantage of this feature is that a shoulder prosthesis of a desired size and shape may be assembled from the kit at the time of the operation. Another advantage results from the large number of varying configuration shoulder prostheses that can be assembled from the component parts without needing to increase the number of assembled shoulder prostheses maintained in hospital inventory.
Another preferred embodiment of the present invention provides for an implant apparatus for replacing a portion of a humerus that includes a body formed to define a bore therein. A head member sized to replace the head of the humerus is eccentrically or centrally attached to the body. Eccentric attachment allows adjustable superior/inferior or anterior/posterior positioning of the head member relative to the body to provide a better fit for the implanted shoulder prosthesis. In preferred embodiments, the eccentric or central attachment is maintained by the interaction of a locking socket defined in the bore of the body to have a polygonal cross section, and a lock finger defined by the head member to present a lock element configured to fit in the locking socket. The locking socket can be formed to have a hexagonal cross section and the lock element of the lock finger hexagonally configured to fit in the locking socket so that the head member can be positioned relative to the body in one of six possible orientations.
The modular shoulder prosthesis of the present invention thus provides the ability to assemble a custom prosthesis by selecting different sizes, shapes, and orientations of individual components to meet the requirements of the individual patient exactly. The provision of the kit greatly reduces the inventory required to be maintained by the hospital. Also, the kit form of the prosthesis increases greatly the flexibility of the implant apparatus, and provides for the assembly of a prosthesis that may otherwise be unavailable.
Additional objects, features, and advantages of the invention will be apparent to those skilled in the art upon consideration of the following detailed description of preferred embodiments exemplifying the best mode of carrying out the invention as presently perceived.